[PATCH] mac80211: bss_tim_clear must use ~ instead of !

We need to use bitwise NOT.
This also cleans up the code a little bit to make it more readable.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>